108 
we collect, that plants, which vegetate in sunshine, 
require always the presence of oxygen gas (309.) ; 
and that, by the act of vegetation, they constantly 
change this oxygen (308.) into carbonic acid. We 
farther learn, that carbonic acid enters plants, both 
with the fluids which they absorb (318.), and also, 
under certain circumstances, in an elastic form 
(300.) ; that this acid gas is conveyed to the leaves, 
and is there decomposed (290. et seq.), by the joint 
operation of the plant and of solar light ; and that it 
is from this source aloire (317.)> tnat tne oxygen gas 
afforded by plants is derived. It likewise appears, 
that this operation of affording oxygen is not proper- 
ly a vegetative function (338.), but only a subordi- 
nate office, accomplished by the direct agency of the 
sun ; that it is carried on in the cellular or parenchy- 
matous structure (323.), and not in the vascular sys- 
tem of the leaf; and that it may, and does exist 
(308.) with that function by which oxygen is con- 
sumed, and which is essential to the vegetation of the 
plant. Hence it is, that, when plants are made to 
grow in closed vessels exposed to the sun, the oxy- 
gen gas which is consumed by the function of vege- 
tation, is again restored (308.) by the decomposition 
of the acid that is formed, and no change (306.), 
therefore, appears to be effected in the composition 
of the air. But in situations, where the direct agency 
of light is excluded (263.), no decomposition of car- 
bonic acid is perceptible, and the air, therefore, soon 
becomes unfit to sustain vegetation. In its general 
nature and effects, therefore, the function of vegeta- 
tion is precisely the same ia sunshine and in the , 
